The present invention relates to gas sensors. More particularly, the present invention relates to a gas sensor with a crimp design.
Exhaust gas sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust gas sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the exhaust sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known gas partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (emf) is developed between the electrodes on the opposite surfaces of the electrolyte wall, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                  where:                                              E          =                      electromotive force                                                        R          =                      universal gas constant                                                        F          =                      Faraday constant                                                        T          =                      absolute temperature of the gas                                                                    P                          O              2                        ref                    =                      oxygen partial pressure of the reference gas                                                                    P                          O              2                                =                      oxygen partial pressure of the exhaust gas                              
Due to the large difference in oxygen partial pressures between fuel-rich and fuel-lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
One known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. The flat plate sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, typically have a high modulus of elasticity and cannot withstand much bending. Hence, great care and expense is expended in preventing manufacturing failures.
Accordingly, there remains a need in the art for a low cost, temperature resistant sensor package.
The drawbacks and disadvantages of the prior art are overcome by the gas sensor and method for making the same. The gas sensor comprises a sensor element; a shell disposed around at least a portion of the sensor element, the shell having a projecting edge from a first end, wherein a portion of the projecting edge is bent toward the sensing element; an upper shield disposed around at least a portion of the sensor element, the upper shield having a terminal end; a gasket disposed between the projecting edge and the terminal end, wherein the gasket is a U-type gasket; and a lower shield affixed to a second end of the shell.
The method of forming the gas sensor comprises providing a shell having a projecting edge from a first end and a segment, wherein the segment is substantially perpendicular to the projecting edge; providing an upper shield having a terminal end with a first side and a second side, wherein the first side of the terminal end is positioned adjacent to the segment; positioning a gasket on the second side of the terminal end, wherein the gasket is a U-type gasket; forming a bent portion of the shell by bending at least a portion of the projecting edge of the shell about the gasket and the terminal end; affixing a lower shield to a second end of the shell; and extending a sensor element through the upper shield, through the shell into the lower shield.
The above described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.